Rear brakes lock up before fronts

[JMD]

Hey all,
I did a rear disc conversion on an HO52 axle for my ‘68 k10. The front axle is a Dana 44 from a mid ‘70s truck that also has disks. I used front calipers from a ‘75 k20 on all four corners.
The proportioning valve is made for a 4 wheel disk conversion and has 1/2” fittings for the rear and 3/8” fittings for the front.
The brakes work fine, but when I really have to stop quickly the rear tires lock up way before the front. In fact, I don’t think I can get the front tires to lock up at all. And worse yet...I can’t even power brake the thing because the rear brakes grab so hard.
I attached a picture of the way I have everything plumbed. I know I can insert an adjustable proportioning valve to reduce the amount of braking power to the rear, but that’s not really the “correct” way of doing things. So what combination of parts do I need to fix this annoying issue?